/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;
import static com.google.common.collect.ObjectArrays.checkElementsNotNull;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Predicate;
import javax.annotation.Nullable;

/**
 * A {@link Collection} whose contents will never change, and which offers a few additional
 * guarantees detailed below.
 *
 * <p>
 * <b>Warning:</b> avoid <i>direct</i> usage of {@link ImmutableCollection} as a type (just as with
 * {@link Collection} itself). Prefer subtypes such as {@link ImmutableSet} or
 * {@link ImmutableList}, which have well-defined {@link #equals} semantics, thus avoiding a common
 * source of bugs and confusion.
 *
 * <h3>About <i>all</i> {@code Immutable-} collections</h3>
 *
 * <p>
 * The remainder of this documentation applies to every public {@code Immutable-} type in this
 * package, whether it is a subtype of {@code ImmutableCollection} or not.
 *
 * <h4>Guarantees</h4>
 *
 * <p>
 * Each makes the following guarantees:
 *
 * <ul>
 * <li><b>Shallow immutability.</b> Elements can never be added, removed or replaced in this
 * collection. This is a stronger guarantee than that of {@link Collections#unmodifiableCollection},
 * whose contents change whenever the wrapped collection is modified.
 * <li><b>Null-hostility.</b> This collection will never contain a null element.
 * <li><b>Deterministic iteration.</b> The iteration order is always well-defined, depending on how
 * the collection was created (see the appropriate factory method for details). View collections
 * such as {@link ImmutableMultiset#elementSet} iterate in the same order as the parent, except as
 * noted.
 * <li><b>Thread safety.</b> It is safe to access this collection concurrently from multiple
 * threads.
 * <li><b>Integrity.</b> This type cannot be subclassed outside this package (which would allow
 * these guarantees to be violated).
 * </ul>
 *
 * <h4>"Interfaces", not implementations</h4>
 *
 * <p>
 * Each public class, such as {@link ImmutableSet}, is a <i>type</i> offering meaningful behavioral
 * guarantees -- not merely a specific <i>implementation</i> as in the case of, say,
 * {@link ArrayList}. You should treat them as interfaces in every important sense of the word.
 *
 * <p>
 * For field types and method return types, you should generally use the immutable type (such as
 * {@link ImmutableList}) instead of the general collection interface type (such as {@link List}).
 * This communicates to your callers all of the semantic guarantees listed above, which is almost
 * always very useful information.
 *
 * <p>
 * On the other hand, a <i>parameter</i> type of {@link ImmutableList} is generally a nuisance to
 * callers. Instead, accept {@link Iterable} and have your method or constructor body pass it to the
 * appropriate {@code copyOf} method itself.
 *
 * <h4>Creation</h4>
 *
 * <p>
 * Except for logically "abstract" types like {@code ImmutableCollection} itself, each {@code
 * Immutable} type provides the static operations you need to obtain instances of that type. These
 * usually include:
 *
 * <ul>
 * <li>Static methods named {@code of}, accepting an explicit list of elements or entries.
 * <li>Static methods named {@code copyOf} (or {@code copyOfSorted}), accepting an existing
 * collection whose contents should be copied.
 * <li>A static nested {@code Builder} class which can be used to populate a new immutable instance.
 * </ul>
 *
 * <h4>Warnings</h4>
 *
 * <ul>
 * <li><b>Warning:</b> as with any collection, it is almost always a bad idea to modify an element
 * (in a way that affects its {@link Object#equals} behavior) while it is contained in a collection.
 * Undefined behavior and bugs will result. It's generally best to avoid using mutable objects as
 * elements at all, as many users may expect your "immutable" object to be <i>deeply</i> immutable.
 * </ul>
 *
 * <h4>Performance notes</h4>
 *
 * <ul>
 * <li>Implementations can be generally assumed to prioritize memory efficiency, then speed of
 * access, and lastly speed of creation.
 * <li>The {@code copyOf} methods will sometimes recognize that the actual copy operation is
 * unnecessary; for example, {@code copyOf(copyOf(anArrayList))} should copy the data only once.
 * This reduces the expense of habitually making defensive copies at API boundaries. However, the
 * precise conditions for skipping the copy operation are undefined.
 * <li><b>Warning:</b> a view collection such as {@link ImmutableMap#keySet} or
 * {@link ImmutableList#subList} may retain a reference to the entire data set, preventing it from
 * being garbage collected. If some of the data is no longer reachable through other means, this
 * constitutes a memory leak. Pass the view collection to the appropriate {@code copyOf} method to
 * obtain a correctly-sized copy.
 * <li>The performance of using the associated {@code Builder} class can be assumed to be no worse,
 * and possibly better, than creating a mutable collection and copying it.
 * <li>Implementations generally do not cache hash codes. If your element or key type has a slow
 * {@code hashCode} implementation, it should cache it itself.
 * </ul>
 *
 * <h4>Example usage</h4>
 *
 * <pre>
 * {
 *     &#64;code
 *
 *     class Foo {
 *         private static final ImmutableSet<String> RESERVED_CODES = ImmutableSet.of("AZ", "CQ", "ZX");
 *
 *         private final ImmutableSet<String> codes;
 *
 *         public Foo(Iterable<String> codes) {
 *             this.codes = ImmutableSet.copyOf(codes);
 *             checkArgument(Collections.disjoint(this.codes, RESERVED_CODES));
 *         }
 *     }
 * }
 * </pre>
 *
 * <h3>See also</h3>
 *
 * <p>
 * See the Guava User Guide article on
 * <a href= "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable
 * collections</a>.
 *
 * @since 2.0
 */
@GwtCompatible(emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
// TODO(kevinb): I think we should push everything down to "BaseImmutableCollection" or something,
// just to do everything we can to emphasize the "practically an interface" nature of this class.
public abstract class ImmutableCollection<E> extends AbstractCollection<E> implements Serializable {
    /*
     * We expect SIZED (and SUBSIZED, if applicable) to be added by the spliterator factory methods.
     * These are properties of the collection as a whole; SIZED and SUBSIZED are more properties of
     * the spliterator implementation.
     */
    static final int SPLITERATOR_CHARACTERISTICS = Spliterator.IMMUTABLE | Spliterator.NONNULL | Spliterator.ORDERED;

    ImmutableCollection() {}

    /**
     * Returns an unmodifiable iterator across the elements in this collection.
     */
    @Override
    public abstract UnmodifiableIterator<E> iterator();

    @Override
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, SPLITERATOR_CHARACTERISTICS);
    }

    private static final Object[] EMPTY_ARRAY = {};

    @Override
    public final Object[] toArray() {
        int size = size();
        if (size == 0) {
            return EMPTY_ARRAY;
        }
        Object[] result = new Object[size];
        copyIntoArray(result, 0);
        return result;
    }

    @CanIgnoreReturnValue
    @Override
    public final <T> T[] toArray(T[] other) {
        checkNotNull(other);
        int size = size();
        if (other.length < size) {
            other = ObjectArrays.newArray(other, size);
        } else if (other.length > size) {
            other[size] = null;
        }
        copyIntoArray(other, 0);
        return other;
    }

    @Override
    public abstract boolean contains(@Nullable Object object);

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    public final boolean add(E e) {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    public final boolean remove(Object object) {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    public final boolean addAll(Collection<? extends E> newElements) {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    public final boolean removeAll(Collection<?> oldElements) {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    public final boolean removeIf(Predicate<? super E> filter) {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @Deprecated
    @Override
    public final boolean retainAll(Collection<?> elementsToKeep) {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @Deprecated
    @Override
    public final void clear() {
        throw new UnsupportedOperationException();
    }

    /**
     * Returns an {@code ImmutableList} containing the same elements, in the same order, as this
     * collection.
     *
     * <p>
     * <b>Performance note:</b> in most cases this method can return quickly without actually
     * copying anything. The exact circumstances under which the copy is performed are undefined and
     * subject to change.
     *
     * @since 2.0
     */
    public ImmutableList<E> asList() {
        switch (size()) {
            case 0:
                return ImmutableList.of();
            case 1:
                return ImmutableList.of(iterator().next());
            default:
                return new RegularImmutableAsList<E>(this, toArray());
        }
    }

    /**
     * Returns {@code true} if this immutable collection's implementation contains references to
     * user-created objects that aren't accessible via this collection's methods. This is generally
     * used to determine whether {@code copyOf} implementations should make an explicit copy to
     * avoid memory leaks.
     */
    abstract boolean isPartialView();

    /**
     * Copies the contents of this immutable collection into the specified array at the specified
     * offset. Returns {@code offset + size()}.
     */
    @CanIgnoreReturnValue
    int copyIntoArray(Object[] dst, int offset) {
        for (E e : this) {
            dst[offset++] = e;
        }
        return offset;
    }

    Object writeReplace() {
        // We serialize by default to ImmutableList, the simplest thing that works.
        return new ImmutableList.SerializedForm(toArray());
    }

    /**
     * Abstract base class for builders of {@link ImmutableCollection} types.
     *
     * @since 10.0
     */
    public abstract static class Builder<E> {
        static final int DEFAULT_INITIAL_CAPACITY = 4;

        static int expandedCapacity(int oldCapacity, int minCapacity) {
            if (minCapacity < 0) {
                throw new AssertionError("cannot store more than MAX_VALUE elements");
            }
            // careful of overflow!
            int newCapacity = oldCapacity + (oldCapacity >> 1) + 1;
            if (newCapacity < minCapacity) {
                newCapacity = Integer.highestOneBit(minCapacity - 1) << 1;
            }
            if (newCapacity < 0) {
                newCapacity = Integer.MAX_VALUE;
                // guaranteed to be >= newCapacity
            }
            return newCapacity;
        }

        Builder() {}

        /**
         * Adds {@code element} to the {@code ImmutableCollection} being built.
         *
         * <p>
         * Note that each builder class covariantly returns its own type from this method.
         *
         * @param element the element to add
         * @return this {@code Builder} instance
         * @throws NullPointerException if {@code element} is null
         */
        @CanIgnoreReturnValue
        public abstract Builder<E> add(E element);

        /**
         * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
         *
         * <p>
         * Note that each builder class overrides this method in order to covariantly return its own
         * type.
         *
         * @param elements the elements to add
         * @return this {@code Builder} instance
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        public Builder<E> add(E... elements) {
            for (E element : elements) {
                add(element);
            }
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
         *
         * <p>
         * Note that each builder class overrides this method in order to covariantly return its own
         * type.
         *
         * @param elements the elements to add
         * @return this {@code Builder} instance
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        public Builder<E> addAll(Iterable<? extends E> elements) {
            for (E element : elements) {
                add(element);
            }
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableCollection} being built.
         *
         * <p>
         * Note that each builder class overrides this method in order to covariantly return its own
         * type.
         *
         * @param elements the elements to add
         * @return this {@code Builder} instance
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        public Builder<E> addAll(Iterator<? extends E> elements) {
            while (elements.hasNext()) {
                add(elements.next());
            }
            return this;
        }

        /**
         * Returns a newly-created {@code ImmutableCollection} of the appropriate type, containing
         * the elements provided to this builder.
         *
         * <p>
         * Note that each builder class covariantly returns the appropriate type of
         * {@code ImmutableCollection} from this method.
         */
        public abstract ImmutableCollection<E> build();
    }

    abstract static class ArrayBasedBuilder<E> extends ImmutableCollection.Builder<E> {
        Object[] contents;
        int size;

        ArrayBasedBuilder(int initialCapacity) {
            checkNonnegative(initialCapacity, "initialCapacity");
            this.contents = new Object[initialCapacity];
            this.size = 0;
        }

        /**
         * Expand the absolute capacity of the builder so it can accept at least the specified
         * number of elements without being resized.
         */
        private void ensureCapacity(int minCapacity) {
            if (contents.length < minCapacity) {
                this.contents = Arrays.copyOf(this.contents, expandedCapacity(contents.length, minCapacity));
            }
        }

        @CanIgnoreReturnValue
        @Override
        public ArrayBasedBuilder<E> add(E element) {
            checkNotNull(element);
            ensureCapacity(size + 1);
            contents[size++] = element;
            return this;
        }

        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E... elements) {
            checkElementsNotNull(elements);
            ensureCapacity(size + elements.length);
            System.arraycopy(elements, 0, contents, size, elements.length);
            size += elements.length;
            return this;
        }

        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterable<? extends E> elements) {
            if (elements instanceof Collection) {
                Collection<?> collection = (Collection<?>) elements;
                ensureCapacity(size + collection.size());
            }
            super.addAll(elements);
            return this;
        }

        @CanIgnoreReturnValue
        ArrayBasedBuilder<E> combine(ArrayBasedBuilder<E> builder) {
            checkNotNull(builder);
            ensureCapacity(size + builder.size);
            System.arraycopy(builder.contents, 0, this.contents, size, builder.size);
            size += builder.size;
            return this;
        }
    }
}
